Solid state image sensors are well known, as they find a widespread use in camera systems. Commonly solid state image sensors are implemented in a CCD-technology or in a CMOS- or MOS-technology. In this embodiment, a matrix of light sensitive elements (photosensitive elements) in series with switching elements constitutes an image sensor, which is mounted in a camera system. The light sensitive elements may for example be photoreceptors, photo-diodes, phototransistors, or alike. Each light sensitive element receives an image of a portion of a scene being imaged. That portion is called a picture element or pixel. The image obtaining light sensitive elements produce an electrical signal indicative of the light intensity of the image. The electrical signal of a light sensitive element is typically a current, which is proportional to the amount of electromagnetic radiation (light) falling onto that light sensitive element.
The signal of said matrix of pixels is measured and multiplexed to a so-called video-signal.
Of the image sensors implemented in a CMOS- or MOS-technology, image sensors with passive pixels and image sensors with active pixels are distinguished. The difference between these two types of pixel structures is that a passive pixel does not perform signal amplification whereas an active pixel does.
A passive pixel sensor is simply a photodiode (MOS or p-n junction diode) with a transistor acting as a switch that passes photo-electrically generated signal charge to an amplifier outside the pixel array.
The term “active pixel” refers to any pixel that has an active element integrated in the pixel, that is, at least one amplifier that typically comprises one or more transistors to amplify the charge that is collected on the light sensitive element in the pixel. Active pixels may also be equipped with additional electronics for more elaborate functions, such as filtering, high speed operation, or operation in more extreme illumination conditions. A commonly used CMOS active pixel sensor (APS) cell is represented in FIG. 1. It has four transistors T1, T2, T3, T4 and a photosensitive element D1. The cell has a transfer gate TG at a transfer transistor T2, separating the photosensitive element D1 from a capacitive ‘floating diffusion’ (which acts as a sample-and-hold capacitor), a reset gate RG at a reset transistor T1 between the floating diffusion and a power supply VDD, a source-follower transistor T4 to buffer the floating diffusion from a readout-line capacitance, and a row-select gate RSG at a row select transistor T3 to connect the cell to the readout line. All pixels on a column connect to a common sense amplifier A.
In the active pixel sensor cell represented, as well as in many other APS cells, column buses are readout as a switched source follower: the driver transistor T4 is in the pixels, and the load transistor is common for all pixels of a column. A select MOSFET T3 acting as a switch connects the pixels to the column.
A simple image-capture cycle for the four-transistor active pixel sensor cell mentioned above is as follows. First the reset gate RG and the transfer gate TG are turned on to reset the photosensitive element D1 and floating diffusion potentials. Both gates RG, TG are then turned off. The photosensitive elements D1 on the pixels convert photons (light) into charge, and these light-induced electrons collect on the photosensitive elements D1. After the desired integration period, the transfer gate TG is turned on, and collected charge transfers to the floating diffusion capacitance. The resultant floating diffusion voltage charge appears on the source-follower output of transistor T4, which is read by connecting it to a readout line via the row-select gate RSG of row-select transistor T3. This cycle is repeated for each pixel capturing an image, and is repeated for each next image.
A classic way to read out an image sensor is line by line. A conventional signal readout scheme of a CMOS image sensor is shown in FIG. 2.
A horizontal scan register Y-addressing addresses all pixels of a line to be read out, whereby all row select switching elements of pixels of that line are closed at the same time. Therefore, each of those pixels puts a signal on a vertical output line V, where it is amplified in column amplifier A1. A vertical scan register X-addressing switches and multiplexes onto an output bus O the charges that have been put on the vertical output lines V.
Between reading out two subsequent lines is a so-called “blanking time” or “blanking period”, which is a time period needed to do some image sensor housekeeping tasks, such as for example, but not limited thereto:                changing the line address; as this involves the charging or discharging of reasonably long lines, this time is certainly not infinitely short;        linewise parallel sampling and holding charges integrated in all pixels of a line;        resetting of the latest line that was selected, in order to allow a new integration period to begin in that line;        resetting of a different line than the one that was previously selected; this operation is often used to perform an “electronic shutter”, which is the ability to control the integration time of a light sensitive pixel;        subtracting the signal of a line and its reset level; this function is often used for fixed pattern noise cancellation.        
In any case, there is a finite delay between the readout of the last pixel of the previous line and the first pixel of the next line, as the line address needs be changed. In the fastest possible configuration known in the prior art, the only function retained during he blanking period is the change of the line address, which corresponds to a change of the selected line to be read out.
For source followers, as in the pixel embodiment of FIG. 1, it is well known that their rise time is fast, but that their fall time is slow, as this is dictated by the discharge of the bus capacitance through the load MOSFET A, which acts as a current source. Therefore, as mentioned previously, even if change of the line address is the only function retained, as this involves the charging or discharging of reasonably long lines, this time is certainly not infinitely short. This discharge time is often the practical limitation on the time needed to change a line in an APS image sensor. Increasing the load current can speed up the discharge time, but this has practical limitations. The driver MOSFETs are inside the pixels (e.g. transistor T4 in FIG. 1), and are thus limited in area and therefore in driving power. If the load current is too large, the driver MOSFET in the pixels cannot counteract it, and the signal level on the bus collapses. Furthermore, all the load currents in parallel for all columns of the image sensor may represent a serious power dissipation, which may cause unacceptable heating of the chip or short battery life.
A schematic representation of the conventional succession of pixel read out sequences and blanking periods is shown in FIG. 3. During blanking time A, the line address is changed so as to set line k as the next line to be read out, and possibly other tasks are performed during blanking time A as well. Thereafter all pixels of line k are read out and multiplexed on an output bus. Once all pixels of line k have been read out, a blanking time B occurs, during which the line address is changed so as to set line k+1 as the next line to be read out. Possibly other tasks are performed during blanking time B as well. Thereafter all pixels of line k+1 are read out and multiplexed on an output bus.